Hydraulic circuit for a construction machine

ABSTRACT

Problem to be solved: To provide a hydraulic circuit for the construction machine which enables to use the relief valve of low capacity in the work tool circuit. Solution: The hydraulic circuit  2  for a construction machine has: a hydraulic pump  4  of variable capacity, a work tool  6  operated by hydraulic oil delivered by the hydraulic pump  4 , a work tool operating device  10  to output a signal for operating the work tool  6 , a control valve  14  allowing the hydraulic pump  4  to supply the hydraulic oil to the work tool  6  based on the signal output from the work tool operating device  10 , a tool&#39;s relief valve  44  to release the hydraulic oil flowing between the control valve  14  and the work tool  6 , a pressure sensor  46  to detect a pressure of hydraulic oil flowing into the work tool  6 , and a controller  48  to reduce a delivery rate from the hydraulic pump  4  when the pressure detected by the pressure sensor  46  exceeds a predetermined value.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 USC § 119 and the ParisConvention to Japanese Patent Application 2021-174755 filed on Oct. 26,2021.

FIELD OF THE INVENTION

The present invention relates to hydraulic circuit for a constructionmachine where various work tools are detachably installed.

BACKGROUND OF THE INVENTION

A hydraulic excavator as a typical example of construction machines hasa lower traveling body, an upper swiveling body swivelably supported onthe lower traveling body, and a front working machine installed on theupper swiveling body. A front working machine for hydraulic excavatorincludes a boom coupled swingably to an upper swiveling body, an armcoupled swingably to a distal end of the boom, and a work tool installeddetachably on the distal end of the arm.

Many hydraulic excavators are installed with a bucket for excavationwork as the work tool, and various work tools may be installed besidesthe bucket. A hydraulic hammer for crushing concrete, rocks, and so on,and a grapple for grabbing wood and so on are examples of work toolsbesides the bucket (see PTL 1, for example).

PRIOR ART DOCUMENT Patent Document

PTL 1: Japanese Unexamined Patent Application Publication No.2010-168738

SUMMARY OF THE INVENTION Problems to be Solved by Invention

Such a work tool includes those which release the hydraulic oil througha relief valve from the circuit (work tool circuit) to operate the worktool (grapple, for example). When using such a work tool, a relief valveof large capacity is needed to be installed in a work tool circuit todeliver a full flow from a pump during the relief. However, the reliefvalve of large capacity is expensive, causing a cost increase in thework tool circuit.

Also, some work tools are required to set a relief pressure lower thanthe relief pressure of the entire circuit for the hydraulic excavator.When using this work tool and any other actuator than the work toolsimultaneously, the pressure of the entire circuit rises only up to therelief pressure of the work tool, causing a problem that an operabilityof any other actuator is degraded.

In consideration of what mentioned above, a first challenge of thepresent invention is to provide a hydraulic circuit for the constructionmachine which enables to use the relief valve of low capacity in thework tool circuit. Also, a second challenge of the present invention isto provide the hydraulic circuit with excellent operability whenoperating the work tool and any other actuator simultaneously.

Means for Solving the Problem

According to a first aspect of this invention, a hydraulic circuit isprovided to the construction machine which solves the first challengementioned above, as follows. That is,

-   -   “A hydraulic circuit for a construction machine comprising:    -   a hydraulic pump of variable capacity,    -   a work tool operated by hydraulic oil delivered by the hydraulic        pump,    -   a work tool operating device to output a signal for operating        the work tool,    -   a control valve allowing the hydraulic pump to supply hydraulic        oil to the work tool based on the signal output from the work        tool operating device,    -   a tool's relief valve to release hydraulic oil flowing between        the control valve and the work tool,    -   a pressure sensor to detect a pressure of hydraulic oil flowing        into the work tool, and    -   a controller which reduces a delivery rate from the hydraulic        pump when the pressure detected by the pressure sensor exceeds a        predetermined value.”

Preferably, the predetermined value is set to not more than the reliefpressure of the tool's relief valve. The control valve appropriately hasa meter-in valve which controls hydraulic oil flow rate flowing into thework tool and a meter-out valve which controls hydraulic oil flow rateflowing out of the work tool.

According to a second aspect of this invention, the hydraulic circuitfor the construction machine is provided which solves the secondchallenge mentioned above, as follows. That is,

-   -   “A hydraulic circuit for a construction machine comprising:    -   a hydraulic pump,    -   a work tool and an actuator respectively operated by hydraulic        oil delivered by the hydraulic pump,    -   a work tool operating device to output a signal for operating        the work tool,    -   an actuator operating device to output the signal for operating        the actuator,    -   a control valve allowing the hydraulic pump to supply hydraulic        oil to the work tool and the actuator based on the signal output        from the work tool and actuator operating devices,    -   a main relief valve disposed at an upstream side of the control        valve to release hydraulic oil delivered by the hydraulic pump,    -   a tool's relief valve to release hydraulic oil flowing between        the control valve and the work tool, and    -   a controller to control an operation of the control valve,    -   wherein, the control valve includes meter-in/meter-out valves        which control hydraulic oil flow rate flowing into/out of the        work tool,    -   wherein, when the signal is output from both the work tool and        actuator operating devices, the controller reduces an opening        area of the meter-in valve so that the pressure at a downstream        side of the meter-in valve is made lower than that at the        upstream side of the meter-in valve.”

Desirably, when the signal is output from both the work tool andactuator operating devices, the controller reduces the opening area ofthe meter-in valve to make the pressure at the downstream side of themeter-in valve lower than that at the upstream side of the meter-invalve lest the pressure at the downstream side of the meter-in valveshould reach to the relief pressure of the tool's relief valve beforethe pressure at the upstream side of the meter-in valve reaches to therelief pressure of the main relief valve.

The hydraulic pump is a variable capacity type and includes the pressuresensor to detect the pressure of hydraulic oil flowing into the worktool, and the controller advantageously reduces the delivery rate fromthe hydraulic pump when the pressure detected by the pressure sensorexceeds the predetermined value.

Both first and second aspects of this invention include first and secondconduits respectively connecting the control valve and the work tool,first and second relief conduits respectively branched from the firstand second conduits to a hydraulic tank, and the tool's relief valve ispreferably provided in each of the first and second relief conduits.Alternatively, these aspects include first and second conduitsrespectively connecting the control valve and the work tool, aconnecting conduit connecting the first and second conduits, a shuttlevalve disposed in the connecting conduit, and the relief conduitextending from an exit of the shuttle valve to the hydraulic tank, andthe tool's relief valve may be provided in the relief conduit.

The tool's relief valve is appropriately electromagnetic proportionalrelief valve.

Favorable Effects of the Invention

According to the first aspect of this invention, the delivery rate isreduced from the hydraulic pump when the pressure of hydraulic oilflowing into the work tool exceeds the predetermined value, so that thehydraulic oil flow rate may be suppressed passing through the tool'srelief valve and the capacity of the tool's relief valve may be reduced.

According to the second aspect of this invention, when the signal isoutput from both the work tool and actuator operating devices, thecontroller reduces an opening area of the meter-in valve to make thepressure at the downstream side of the meter-in valve lower than that atthe upstream side of the meter-in valve, so the pressure of hydraulicoil flowing into the actuator can be raised higher than the reliefpressure of the tool's relief valve even if the relief pressure of thetool's relief valve is set lower than that of the main relief valve, andthe operability becomes better when operating the work tool and actuatorsimultaneously.

Also, as the second aspect of this invention has the meter-in/meter-outvalves which control hydraulic oil flow rate flowing into/out of thework tool, there is no need to reduce the opening of the meter-out valvein conjunction with reducing the opening of the meter-in valve, enablingto suppress back pressure increase in the work tool circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram for the construction machineconfigured according to this invention.

FIG. 2 is a first circuit diagram variation of the hydraulic circuitshown in FIG. 1 .

FIG. 3 is the second circuit diagram variation of the hydraulic circuitshown in FIG. 1 .

FIG. 4 is the third circuit diagram variation of the hydraulic circuitshown in FIG. 1 .

DETAILED DESCRIPTION OF THE INVENTION

Now, an embodiment of the hydraulic circuit for the construction machineconfigured according to the present invention will be described withreference to the drawings above.

[Hydraulic Circuit 2]

As shown in FIG. 1 , the hydraulic circuit 2, which may be mounted onthe construction machine like hydraulic excavator, has the hydraulicpump 4 of variable capacity, the work tool 6 and actuator 8 bothoperated by hydraulic oil (delivery oil) delivered by the hydraulic pump4, the work tool operating device 10 to output the signal to operate thework tool 6, the actuator operating device 12 to output the signal tooperate the actuator 8, the control valve 14 allowing the hydraulic pump4 to supply the hydraulic oil to the work tool 6 and the actuator 8based on the signal output from the work tool and actuator operatingdevices 10, 12, and first and second conduits 16, 18 respectivelyconnecting the control valve 14 and the work tool 6.

(Hydraulic Pump 4)

The hydraulic pump 4 is driven by an engine 20 to suck hydraulic oilfrom the hydraulic tank 22 and deliver it to a pump conduit 24. Asdepicted in FIG. 1 , the pump conduit 24 connects hydraulic pump 4 andcontrol valve 14. Also, as mentioned above, the hydraulic pump 4delivers hydraulic oil to the work tool 6 and actuator 8, but a pilotpump may be provided separately.

(Work Tool 6)

The work tool 6 installed detachably in the hydraulic circuit 2 iseither single-acting or double-acting work tool 6 a or 6 b (see FIGS. 1,2 , respectively). The single-acting work tool 6 a depicted in FIG. 1uses either of first and second conduits 16, 18 as an inflow route onlyand the other as outflow route only. As an example of single-acting worktool 6 a, there is a hydraulic hammer for crushing concrete or stones.

Meanwhile, the double-acting work tool 6 b as depicted in FIG. 2 usesboth first and second conduits 16, 18 alternately as inflow and outflowroutes. As the example of double-acting work tool 6 b, there is thegrapple for grabbing wood, etc., for example. The actuator for thedouble-acting work tool 6 b may be either hydraulic cylinder or motor.

(Actuator 8)

The actuator 8 is illustrated as the hydraulic cylinder in FIGS. 1, 2 ,but it is not limited to the hydraulic cylinder and may be the hydraulicmotor. Only one actuator 8 is shown in the illustrated embodiments, buttwo or more actuators 8 may be provided. As examples of the actuator 8,when the hydraulic circuit 2 is for hydraulic excavator, there areboom/arm cylinders swinging a boom/arm, a traveling motor travelinghydraulic excavator, a swiveling motor swiveling upper swiveling body,and others.

(Work Tool and Actuator Operating Devices 10, 12)

The work tool and actuator operating devices 10, 12 may be configured tohave an input equipment (joystick, slide switch, pedal, etc., forexample) which increases intensity of an output signal as an operatingamount increases. Only one actuator operating device 12 is shown in theillustrated embodiments, but two or more actuator operating devices 12may be provided.

When operated by an operator, the work tool operating device 10 outputsan electric or hydraulic signal to operate the work tool 6. Also, whenoperated by the operator, the actuator operating device 12 outputs theelectric or hydraulic signal to operate the actuator 8. FIG. 1illustrates a configuration where the work tool and actuator operatingdevices 10, 12 output electric signal. The electric signal output fromthe work tool and actuator operating devices 10, 12 is sent to thecontrol valve 14 via a controller 48 mentioned later.

Note that, unlike the illustrated embodiments, the work tool andactuator operating devices 10, 12 may output hydraulic signal to thecontrol valve 14. Here, a pressure sensor (not shown) detects thehydraulic signal output from the work tool and actuator operatingdevices 10, 12 and inputs detection result into the controller 48.

(Control Valve 14)

According to the illustrated embodiments, the control valve 14 includesmeter-in/meter-out valves 26, 28 which control hydraulic oil flow rateflowing into/out of the work tool 6, actuator valve 30 which controlshydraulic oil flow rate flowing into/out of the actuator 8, a firstcheck valve 32 mounted at the upstream side of the meter-in valve 26,and a second check valve 34 mounted at the upstream side of the actuatorvalve 30.

According to the illustrated embodiments, the meter-in/meter-out valves26, 28 are of electromagnetic proportional type where the opening areaof the valves 26, 28 is controlled by the controller 48 based on theelectric signal output by the work tool operating device 10 to thecontroller 48, but the meter-in/meter-out valves 26, 28 may be ofhydraulic pilot operated type operated by hydraulic signal output fromthe work tool operating device 10.

As depicted in FIG. 1 , the meter-in valve 26 is a two port switchingvalve and is installed in the pump conduit 24. When the work tooloperating device 10 outputs the signal, the meter-in valve 26 is openedby the controller 48.

The meter-out valve 28 is a four port switching valve and is providedbetween the meter-in valve 26 and work tool 6. The meter-out valve 28connects the pump conduit 24 to either one of first and second conduits16, 18 and also connects the other one of the first and second conduits16, 18 to a first return conduit 36 according to the signal output fromthe work tool operating device 10. The first return conduit 36 leadsfrom the meter-out valve 28 to the hydraulic tank 22.

According to the illustrated embodiments, similar to the meter-in valve26 and others, the actuator valve 30 is of electromagnetic proportionaltype where the opening area of the valve 30 is controlled by thecontroller 48 based on the electric signal output to the controller 48from the actuator operating device 12, but the valve 30 may be ofhydraulic pilot operated type operated by hydraulic signal output fromthe actuator operating device 12.

The actuator valve 30 is the four port switching valve and is disposedin the pump conduit 24. The actuator valve 30 connects the pump conduit24 to either one of third and fourth conduits 38, 40 and also connectsthe other one of the third and fourth conduits 38, 40 to a second returnconduit 42 according to the signal output from the actuator operatingdevice 12.

Both the third and fourth conduits 38, 40 connect the actuator valve 30and actuator 8, and the second return conduit 42 leads from the actuatorvalve 30 to the hydraulic tank 22.

As depicted in FIG. 1 , according to the illustrated embodiment, thehydraulic circuit 2 further has the tool's relief valve 44 to releasethe hydraulic oil flowing between the control valve 14 and the work tool6, the pressure sensor 46 to detect the pressure of hydraulic oilflowing into the work tool 6, the controller 48, and first and secondrelief conduits 50, 52 respectively branched from the first and secondconduits 16, 18 to the hydraulic tank 22.

(Tool's Relief Valve 44)

In the illustrated embodiment, the first and second relief conduits 50,52 are provided with the tool's relief valve 44 respectively. The tool'srelief valve 44 is to release the hydraulic oil flowing in the first andsecond relief conduits 50, 52 to the hydraulic tank 22 when the pressureof hydraulic oil in the first and second relief conduits 50, 52 exceedsthe relief pressure.

The relief pressure of the tool's relief valve 44 is set to a requiredvalue (initial value) in advance, but when the tool's relief valve 44 isof electromagnetic proportional type, the initial value above may bemodified appropriately by the controller 48 based on a kind of the worktool 6 input by the operator into the controller 48.

When the tool's relief valve 44 is of electromagnetic proportional type,the controller 48 may change the relief pressure of the tool's reliefvalve 44 not only when the kind of the work tool 6 is input but when therequired signal is output from the work tool operating device 10.

When the signal is output from the work tool operating device 10, forexample, the relief pressure of the tool's relief valve 44 provided atthe outflow route side of the work tool 6 may be relieved. Thus, returnoil from the work tool 6 branches into a route through the meter-outvalve 28 and the route through the tool's relief valve 44 at an outflowroute side, allowing to suppress back pressure increase in the work toolcircuit. Note that, since the tool's relief valve 44 at the inflow routeside of the work tool 6 does not give an impact on suppression of theback pressure increase, there is no need to change the relief pressure.

(Pressure Sensor 46)

The pressure sensor 46 is attached to the meter-out valve 28 and detectsthe pressure of hydraulic oil flowing from the meter-out valve 28 to thework tool 6 in either case when the hydraulic oil flows in first orsecond conduit 16 or 18. The result detected by the pressure sensor 46is sent to the controller 48.

(Controller 48)

The controller 48 is comprised of a computer having processing andstorage devices. The controller 48 controls the operation of the controlvalve 14 as well as the delivery rate of the hydraulic pump 4 based onthe signal output from the work tool and actuator operating devices 10,12.

As depicted in FIG. 1 , the first and second conduits 16, 18 are alsoconnected to the hydraulic tank 22 by way of a make-up conduit 54. Themake-up conduit 54 is disposed with a pair of make-up check valves 56 inorder to avoid a cavitation from being generated in the first and secondconduits 16, 18 when a negative pressure occurs in the passages 16, 18.

According to the illustrated embodiment, the hydraulic circuit 2includes a pressure sensor 58 to detect the pressure of hydraulic oil inthe pump conduit 24, a bypass conduit 60 branched from the pump conduit24 to the hydraulic tank 22, a bypass valve 62 of electromagneticproportional type to control hydraulic oil flow rate returning throughthe bypass conduit 60 to the hydraulic tank 22, and a main relief valve64 disposed at an upstream side of the control valve 14 to release thehydraulic oil delivered by the hydraulic pump 4.

The main relief valve 64 is to release the hydraulic oil flowing in thepump conduit 24 to the hydraulic tank 22 when the pressure of hydraulicoil in the pump conduit 24 exceeds the relief pressure. In general, therelief pressure of the main relief valve 64 is set higher than that ofthe tool's relief valve 44.

As depicted in FIG. 1 , the first and second return conduits 36, 42 bothconnected to the hydraulic tank 22, the first and second relief conduits50, 52, and the make-up conduit 54 are joined together before thehydraulic tank 22. The hydraulic oil is to return to the hydraulic tank22 by flowing through either a cooling conduit 68 via an oil cooler 66or non-cooling conduit 70 not via the oil cooler 66.

Next, an explanation is provided about the operation of hydrauliccircuit 2 for construction machine, as mentioned above.

When the work tool and actuator operating devices 10, 12 are notoperated, the signal is not output from the operating devices 10, 12.Here, the meter-in/meter-out/actuator valves 26, 28, and 30 are closed,the delivery oil from the hydraulic pump 4 does not flow into the worktool 6 and actuator 8, and the work tool 6 and others do not work.

Also, when the work tool operating device 10, etc. is not operated, thecontroller 48 opens the bypass valve 62. Thus, the delivery oil from thehydraulic pump 4 returns to the hydraulic tank 22 through the bypassconduit 60.

(Operation of Work Tool 6)

When the work tool operating device 10 is operated, the operating device10 outputs the signal. Then, the controller 48 operates themeter-in/meter-out valves 26, 28 to open an oil passage from thehydraulic pump 4 to the work tool 6 and also reduce the opening area ofthe bypass valve 62. Thus, the delivery oil is supplied from thehydraulic pump 4 to the work tool 6 to run the work tool 6.

When the amount operated by the work tool operating device 10 increases,the intensity of signal output from the work tool operating device 10rises as the operation amount increases. As the intensity of signal fromthe work tool operating device 10 rises, the controller 48 increases thedelivery rate from the hydraulic pump 4 and the opening area of themeter-in/meter-out valves 26, 28, and also decreases the opening area ofthe bypass valve 62. So, as the amount operated by the work tooloperating device 10 increases, a working speed of the work tool 6 rises.

However, when the pressure (pressure of hydraulic oil flowing from themeter-out valve 28 to the work tool 6) detected by the pressure sensor46 exceeds the predetermined value, the controller 48 reduces thedelivery rate of the hydraulic pump 4 compared to cases where thepressure detected by the pressure sensor 46 is not more than thepredetermined value even if the amount operated by the work tooloperating device 10 does not change.

This allows to suppress the hydraulic oil flow rate flowing through thetool's relief valve 44 at the inflow route side of the work tool 6 whenthe pressure of hydraulic oil exceeds the predetermined value in theinflow route side (first or second conduit 16 or 18) to the work tool 6,enabling to reduce the capacity of the tool's relief valve 44. Thehydraulic oil flow rate is suppressed flowing through the tool's reliefvalve 44, so an energy loss is relieved during relief.

It is preferable that the delivery rate of the hydraulic pump 4 is notmore than the capacity (flow rate allowing to pass through) of thetool's relief valve 44 after the delivery rate is reduced and all thehydraulic oil delivered from the hydraulic pump 4 is able to passthrough the tool's relief valve 44 after reducing the delivery rate.

The predetermined value above to control and reduce the delivery rate ofhydraulic pump 4 may be set to any value not more than the reliefpressure (initial value) of the tool's relief valve 44. However, from apoint of view to reduce the impact on the operation of the work tool 6,the predetermined value above is preferably as large as possible.

When the controller 48 changed the relief pressure of the tool's reliefvalve 44 from the initial value based on the kind of work tool 6, thepredetermined valve above may be changed according to the change of therelief pressure.

However, when the relief pressure of the tool's relief valve 44 isreduced at the outflow route side of the work tool 6 in order tosuppress the back pressure increase in the work tool circuit whileoperating the work tool 6, there is no need to change the predeterminedvalve above according to the reduction of the relief pressure. The abovepredetermined value is to suppress the hydraulic oil flow rate flowingthrough the tool's relief valve 44 at the inflow route side of the worktool 6, and is less related to the relief pressure of the tool's reliefvalve 44 at the outflow route side of the work tool 6.

The controller 48 controls the delivery rate as explained above (controlwhich reduces the delivery rate of the hydraulic pump 4 when thepressure detected by the pressure sensor 46 exceeds the predeterminedvalue) in cases when the signal is output from the work tool operatingdevice 10 only and also when the signal is output from both work tooland actuator operating devices 10, 12.

(Operation of the Work Tool 6 and Actuator 8)

When the work tool and actuator operating devices 10, 12 are operated,the signal is output from both operating devices 10, 12. Then, thecontroller 48 operates the meter-in/meter-out/actuator valves 26, 28,and 30 to open each oil passage from the hydraulic pump 4 to the worktool 6 and actuator 8, and also reduce the opening area of the bypassvalve 62. Thus, the delivery oil is supplied from the hydraulic pump 4to the work tool 6 and actuator 8 to run the work tool 6 and actuator 8.

As explained above, as the strength of the signal output from the worktool operating device 10 increases, the opening area of the meter-invalve 26 is increased by the controller 48. However, when the signal isoutput from both work tool and actuator operating devices 10, 12, theopening area of the meter-in valve 26 is controlled by the controller 48to be smaller as compared when the signal is output only from the worktool operating device 10.

That is, when comparing the case (1) the signal is output only from thework tool operating device 10 and the case (2) the signal is output fromboth work tool and actuator operating devices 10, 12, the controller 48controls to make the opening area of the meter-in valve 26 in the case(1) smaller than that in the case (2) even if the signal output from thework tool operating device 10 has the same strength in both cases (1),(2).

More specifically, when the signal is output from both of work tool andactuator operating devices 10, 12, the controller 48 performs thefollowing control. The pressure at downstream side of the meter-in valve26 is controlled to be less than the pressure at upstream side of themeter-in valve 26 by reducing the opening area of the meter-in valve 26so that the pressure (pressure of hydraulic oil flowing into the worktool 6) at downstream side of the meter-in valve 26 will not reach tothe relief pressure of the tool's relief valve 44 before the pressure(pressure of hydraulic oil flowing into the actuator 8) at upstream sideof the meter-in valve 26 reaches to the relief pressure of the mainrelief valve 64.

Thus, even if the relief pressure of the tool's relief valve 44 is sethigher than that of the main relief valve 64, the pressure of hydraulicoil flowing into the actuator 8 can be raised higher than the reliefpressure of the tool's relief valve 44 (until the relief pressure of themain relief valve 64 in the illustrated embodiment). So, the operabilitybecomes better when operating the work tool 6 and actuator 8simultaneously.

Also, the illustrated embodiment has the meter-in/meter-out valves 26,28 which control hydraulic oil flow rate flowing into/out of the worktool 6, so there is no need to reduce the opening of the meter-out valve28 in conjunction with reducing the opening of the meter-in valve 26,enabling to suppress the back pressure increase in the work toolcircuit.

Note that the present invention can accept various variations withoutbeing limited to the embodiment explained above. The embodimentexplained above has given the example with two tool's relief valves 44,but the tool's relief valve 44 may be one.

Referring to FIG. 2 for illustration of this example, the hydrauliccircuit, the whole of which is indicated by a number 72, includes theconnecting conduit 74 connecting the first and second conduits 16, 18,the shuttle valve 76 disposed in the connecting conduit 74, and therelief conduit 78 extending from the exit of the shuttle valve 76 to thehydraulic tank 22. Single tool's relief valve 44 is provided in therelief conduit 78. The example depicted in FIG. 2 has one tool's reliefvalve 44, so this example can suppress a cost compared to the caseprovided with two tool's relief valves 44 (configuration depicted inFIG. 1 ).

Also, as another variation, the configuration with two hydraulic pumps 4may be provided as shown in FIG. 3 . The hydraulic circuit 80 depictedin FIG. 3 is provided with two hydraulic pumps 4 (4 a, 4 b), and alsoprovided with two pump conduits 24 (24 a, 24 b), two meter-in valves 26(26 a, 26 b), two actuators 8 (8 a, 8 b), two actuator valves 30 (30 a,30 b), two bypass valves 62 (62 a, 62 b), and others.

Also, the hydraulic circuit 80 depicted in FIG. 3 is provided with acoupling conduit 82 coupling first and second pump conduits 24 a, 24 b,a shuttle valve 84 disposed in the coupling conduit 82, and a mainrelief conduit 86 extending from the exit of the shuttle valve 84 to thehydraulic tank 22. The main relief conduit 86 is installed with the mainrelief valve 64.

In FIG. 3 , in order to avoid complicated drawing, the work tool andactuator operating devices 10, 12 and controller 48 are omitted.

When two hydraulic pumps 4 a, 4 b are provided as shown in FIG. 3 , thehydraulic oil is supplied to the work tool 6 from either first hydraulicpump 4 a only, second hydraulic pump 4 b only, or both hydraulic pumps 4a, 4 b.

When both hydraulic pumps 4 a, 4 b supply same volume of hydraulic oilto the work tool 6 respectively, as the intensity of the signal outputfrom the work tool operating device 10 rises, the opening area of eachof bypass valves 62 a, 62 b decreases gradually and the opening area ofeach of meter-in valves 26 a, 26 b increases gradually.

When the pressure detected by the pressure sensor 46 exceeds thepredetermined value which is set to not more than the relief pressure ofthe tool's relief valve 44, the discharge rate from each of hydraulicpumps 4 a, 4 b is reduced.

When the signal is output from both the work tool and actuator operatingdevices 10, 12 in the example shown in FIG. 3 , the controller reducesthe opening area of the meter-in valves 26 a, 26 b to make the pressureat the downstream side of the meter-in valves 26 a, 26 b lower than thatat the upstream side of the meter-in valves 26 a, 26 b lest the pressureat the downstream side of the meter-in valves 26 a, 26 b should reach tothe relief pressure of the tool's relief valve 44 before the pressure atthe upstream side of the meter-in valves 26 a, 26 b reaches to therelief pressure of the main relief valve 64.

When the hydraulic oil is supplied to the work tool 6 from firsthydraulic pump 4 a only, the opening area of first meter-in valve 26 ais controlled and the opening area of second meter-in valve 26 b is keptclosed according to the operation of the work tool operating device 10.

On the contrary, when the hydraulic oil is supplied to the work tool 6from second hydraulic pump 4 b only, the opening area of second meter-invalve 26 b is controlled and the opening area of first meter-in valve 26a is kept closed according to the operation of the work tool operatingdevice 10.

Moreover, similar to the hydraulic circuit 90 shown in FIG. 4 inaddition to the example shown in FIG. 3 , there may be only one tool'srelief valve 44 when two hydraulic pumps 4 (4 a, 4 b) are provided.

What is claimed is:
 1. A hydraulic circuit for a construction machinecomprising: a hydraulic pump of variable capacity, a work tool operatedby hydraulic oil delivered by the hydraulic pump, a work tool operatingdevice to output a signal for operating the work tool, a control valveallowing the hydraulic pump to supply hydraulic oil to the work toolbased on the signal output from the work tool operating device, a tool'srelief valve to release the hydraulic oil flowing between the controlvalve and the work tool, a pressure sensor to detect a pressure ofhydraulic oil flowing into the work tool, a controller to reduce adelivery rate from the hydraulic pump when the pressure detected by thepressure sensor exceeds a predetermined value, first and second conduitsrespectively connecting the control valve and the work tool, a firstrelief conduit branched from the first conduit to a hydraulic tank, anda second relief conduit branched from the second conduit to thehydraulic tank, wherein the tool's relief valve is provided in each ofthe first and second relief conduits.
 2. The hydraulic circuit for theconstruction machine of claim 1, wherein the predetermined value is setto not more than a relief pressure of the tool's relief valve.
 3. Thehydraulic circuit for the construction machine of claim 1, wherein thecontrol valve has a meter-in valve which controls hydraulic oil flowrate flowing into the work tool and a meter-out valve which controlshydraulic oil flow rate flowing out of the work tool.
 4. A hydrauliccircuit for a construction machine comprising: a hydraulic pump ofvariable capacity, a work tool operated by hydraulic oil delivered bythe hydraulic pump, a work tool operating device to output a signal foroperating the work tool, a control valve allowing the hydraulic pump tosupply hydraulic oil to the work tool based on the signal output fromthe work tool operating device, a tool's relief valve to release thehydraulic oil flowing between the control valve and the work tool, apressure sensor to detect a pressure of hydraulic oil flowing into thework tool, a controller to reduce a delivery rate from the hydraulicpump when the pressure detected by the pressure sensor exceeds apredetermined value, first and second conduits respectively connectingthe control valve and the work tool, a connecting conduit connecting thefirst and second conduits, a shuttle valve disposed in the connectingconduit, and the relief conduit extending from an exit of the shuttlevalve to the hydraulic tank, wherein the tool's relief valve is providedin the relief conduit.
 5. The hydraulic circuit for the constructionmachine of claim 1, wherein the tool's relief valve is ofelectromagnetic proportional type.
 6. The hydraulic circuit for theconstruction machine of claim 4, wherein the predetermined value is setto not more than a relief pressure of the tool's relief valve.
 7. Thehydraulic circuit for the construction machine of claim 4, wherein thecontrol valve has a meter-in valve which controls hydraulic oil flowrate flowing into the work tool and a meter-out valve which controlshydraulic oil flow rate flowing out of the work tool.
 8. The hydrauliccircuit for the construction machine of claim 4, wherein the tool'srelief valve is of electromagnetic proportional type.
 9. A hydrauliccircuit for a construction machine comprising: a hydraulic pump, a worktool and an actuator respectively operated by hydraulic oil delivered bythe hydraulic pump, a work tool operating device to output a signal foroperating the work tool, an actuator operating device to output thesignal for operating the actuator, a control valve allowing thehydraulic pump to supply hydraulic oil to the work tool and the actuatorbased on the signal output from the work tool and actuator operatingdevices, a main relief valve disposed at an upstream side of the controlvalve to release the hydraulic oil delivered by the hydraulic pump, atool's relief valve to release the hydraulic oil flowing between thecontrol valve and the work tool, and a controller to control anoperation of the control valve, wherein, the control valve includesmeter-in/meter-out valves which control hydraulic oil flow rate flowinginto/out of the work tool, wherein, when the signal is output from boththe work tool and actuator operating devices, the controller reduces anopening area of the meter-in valve so that the pressure at a downstreamside of the meter-in valve is made lower than that at the upstream sideof the meter-in valve.
 10. The hydraulic circuit for the constructionmachine of claim 9, wherein, when the signal is output from both thework tool and actuator operating devices, the controller reduces theopening area of the meter-in valve to make the pressure at thedownstream side of the meter-in valve lower than that at the upstreamside of the meter-in valve lest the pressure at the downstream side ofthe meter-in valve should reach to the relief pressure of the tool'srelief valve before the pressure at the upstream side of the meter-invalve reaches to the relief pressure of the main relief valve.
 11. Thehydraulic circuit for the construction machine of claim 9, wherein thehydraulic pump is a variable capacity type and includes the pressuresensor to detect the pressure of hydraulic oil flowing into the worktool, and the controller reduces the delivery rate from the hydraulicpump when the pressure detected by the pressure sensor exceeds thepredetermined value.